MESOZOIC AND CENOZOIC GEOGRAPHY 483 
valley of the St. Lawrence was opened, parts of the basin of this 
river, not now occupied by lake water, were drowned by the mar- 
ginal lakes. The normal St. Lawrence channel being forbidden, 
other outlets were found; one of these was past Chicago, another 
by Fort Wayne, Ind., and still a third through the Mohawk in 
New York. As the ice withdrew, and these outlets were one by one 
revealed, the overflow of the Great Lakes successively changed, 
until finally, the present channel past the Thousand Islands was 
Fic. 267. 
Surface of the terminal moraine near Ithaca, N. Y. 
assumed for the outlet. This complex history of the Great 
Lakes is clearly recorded in the ancient beaches which now exist 
upon the land on either side (Fig. 177), and at various elevations 
above the present lake waters. During this time of withdrawal 
of the glaciers, other temporary great lakes were caused in the 
valleys of north-flowing streams, the most notable of which was 
an immense lake formed in the broad valley of the Red River 
of the North. 
